Not applicable.
Not applicable.
Pressure swing adsorption is a well-known method for the separation of bulk gas mixtures and for the purification of gas streams containing low concentrations of undesirable components. The method has been developed and adapted for a wide range of feed gases, operating conditions, product purity, and product recovery. Many pressure swing adsorption systems utilize two or more adsorber beds operated in a cyclic sequence in order to maintain a constant product flow rate while selected beds undergo various steps including adsorption, depressurization, evacuation, purge, pressure equalization, repressurization, and other related steps. Multiple adsorber beds using numerous process steps are required to achieve high purity and/or recovery of valuable gaseous products such as hydrogen, carbon oxides, synthesis gas, light hydrocarbons, and the like. Multiple adsorber beds using these process steps also are used to recover oxygen from air.
Many of these pressure swing adsorption processes operate partially at pressures below atmospheric and are described in the art as vacuum swing adsorption (VSA) or vacuum-pressure swing adsorption (VPSA) processes. In the present specification, pressure swing adsorption (PSA) is used as a generic term to describe all types of cyclic adsorption systems regardless of operating pressure levels.
In pressure swing adsorption process cycles, the gas needed for the purge and repressurization steps is provided by gas obtained during other process steps. Repressurization can be accomplished by using final product gas, intermediate gas obtained by pressure equalization among beds, pressurized feed gas, or combinations thereof. Purge can be provided by intermediate depressurization gas from other beds and/or by final product gas.
Feed repressurization is disclosed in representative U.S. Pat. Nos. 4,406,675 and 5,540,758, and in European Patent Publication No. 0 354 259. The use of product gas for purge and/or repressurization is presented in representative U.S. Pat. Nos. 5,328,503, 5,411,578, 5,429,666, and 5,656,067. U.S. Pat. Nos. 5,330,561 and 5,203,888 disclose bed repressurization using pressurized feed gas or product gas.
It is desirable in pressure swing adsorption processes to minimize or eliminate the need to use final product gas for purge and repressurization. This can be difficult in many cases due to feed gas composition, product purity requirements, and product recovery requirements. The present invention, which is described below and defined by the claims which follow, allows maximum product recovery by not requiring the use of final product gas in purge and repressurization steps. This results in reduced power consumption and capital cost for the process.
The invention is a pressure swing adsorption process for the separation of a pressurized feed gas containing at least one more strongly adsorbable component and at least one less strongly adsorbable component. The process comprises the steps of:
(a) introducing the pressurized feed gas into a feed end of an adsorber bed containing one or more solid adsorbents which preferentially adsorb the more strongly adsorbable component and withdrawing from a product end of the adsorber bed a first adsorber effluent gas enriched in the less strongly adsorbable component, wherein the first adsorber effluent gas is utilized as final product gas;
(b) terminating the introduction of the pressurized feed gas into the adsorber bed while withdrawing from the product end of the adsorber bed a second adsorber effluent gas enriched in the less strongly adsorbable component, wherein the pressure in the adsorber bed decreases while the second adsorber effluent gas is withdrawn therefrom, and wherein the second adsorber effluent gas is utilized as additional final product gas;
(c) depressurizing the adsorber bed to a minimum bed pressure by withdrawing additional gas therefrom;
(d) repressurizing the adsorber bed by introducing repressurization gas into the bed, wherein at least a portion of the repressurization gas is provided by pressurized feed gas and none of the repressurization gas is provided by the final product gas; and
(e) repeating steps (a) through (d) in a cyclic manner.
The adsorber bed can be one of a plurality of adsorber beds, each of which undergoes in turn steps (a) through (e).
The depressurizing of each adsorber bed in step (c) can be accomplished by
(c1) withdrawing a first gas stream from the bed until the pressure therein reaches a first intermediate pressure;
(c2) withdrawing a second gas stream from the bed until the pressure therein reaches a second intermediate pressure; and
(c3) evacuating the bed from the feed end until the pressure therein reaches the minimum bed pressure.
The process can further comprise purging each bed following the evacuation of step (c3) by introducing a purge gas into the product end of the bed while continuing to evacuate gas from the feed end of the bed. The purge gas can be provided to the bed by the first gas stream withdrawn from another bed in step (c1).
The process can further comprise evacuating the bed from the feed end while withdrawing the second gas stream from the product end of the bed during step (c2). A portion of the repressurization gas introduced into a bed can be provided by the second gas stream withdrawn from another bed in step (c2).
The pressurized feed gas can be air, in which case the more strongly adsorbable component is nitrogen, the less strongly adsorbable component is oxygen, and the final product gas is enriched in oxygen. A portion of the repressurization gas in step (d) can be provided by allowing atmospheric air to flow into the adsorber bed when the pressure in the bed is initially below atmospheric pressure.
The adsorber bed optionally can be a single adsorber bed. In this case, the depressurizing of the adsorber bed in step (c) can be accomplished by
(c1) withdrawing a first gas stream from the bed until the pressure therein reaches a first intermediate pressure;
(c2) withdrawing a second gas stream from the bed until the pressure therein reaches a second intermediate pressure; and
(c3) evacuating the bed from the feed end until the pressure therein reaches the minimum bed pressure.
The process can further comprise purging the bed following the evacuation of step (c3) by introducing a purge gas into the product end of the bed while continuing to evacuate gas from the feed end of the bed. At least a portion of the additional gas withdrawn while depressurizing the adsorber by withdrawing gas therefrom in step (c) optionally is introduced into a gas storage tank. The purge gas can be provided to the bed by at least a portion of the gas introduced into the gas storage tank in step (c).
Portions of the first adsorber effluent gas and the second adsorber effluent gas can be introduced into a product gas storage tank during steps (a) and (b), and stored gas can be withdrawn from the product storage tank during steps (c) and (d) to provide final product gas.
In one possible embodiment, the present invention is a pressure swing adsorption process for the separation of a pressurized feed gas containing at least one more strongly adsorbable component and at least one less strongly adsorbable component which comprises the steps of:
(a) introducing the pressurized feed gas into a feed end of an adsorber bed containing one or more solid adsorbents which preferentially adsorbs the more strongly adsorbable component and withdrawing from a product end of the adsorber bed a first adsorber effluent gas enriched in the less strongly adsorbable component, wherein the first adsorber effluent gas is utilized as a final product gas, wherein the adsorber bed is one of a plurality of adsorber beds;
(b) terminating the introduction of the pressurized feed gas into the adsorber bed while withdrawing from the product end of the adsorber bed a second adsorber effluent gas enriched in the less strongly adsorbable component, wherein the pressure in the adsorber bed decreases while the second adsorber effluent gas is withdrawn therefrom, and wherein the second adsorber effluent gas is utilized as a final product gas;
(c) depressurizing the adsorber bed to a minimum bed pressure by withdrawing additional gas therefrom, wherein the depressurizing of the adsorber bed is accomplished by
(c1) withdrawing a first gas stream from the bed until the pressure therein reaches a first intermediate pressure;
(c2) withdrawing a second gas stream from the bed until the pressure therein reaches a second intermediate pressure; and
(c3) evacuating the bed from the feed end until the pressure therein reaches the minimum bed pressure;
(d) purging the bed following the evacuation of step (c3) by introducing a purge gas into the product end of the bed while continuing to evacuate gas from the feed end of the bed, wherein the purge gas is provided to the bed by the first gas stream withdrawn from another bed in step (c1);
(e) repressurizing the adsorber bed by introducing the second gas stream withdrawn from another bed during step (c2);
(f) further repressurizing the adsorber bed by introducing pressurized feed gas into the bed; and
(g) repeating steps (a) through (f) in a cyclic manner; wherein none of the repressurization gas in (e) and (f) is provided by the final product gas.
The pressurized feed gas can be air, in which case the more strongly adsorbable component is nitrogen, the less strongly adsorbable component is oxygen, and the final product gas is enriched in oxygen. The process can further comprise, following step (e) and prior to step (f), allowing atmospheric air to flow into the adsorber bed when the pressure in the bed is initially below atmospheric pressure. The process can further comprise evacuating the bed from the feed end while withdrawing the second gas stream from the product end of the bed during step (c2).